Degradation of EB-PVD thermal barrier coatings caused by CMAS deposits

In aero-turbine engines,thermal barrier coatings(TBCs) must be capable to withstand harsh environments,such as high-temperature oxidation and hot-corrosion.Recently,a new failure mode of TBCs caused by calcium-magnesium-alumina-silicate(CMAS) glass has attracted increasing attention.In this paper,yttria stabilized zirconia(YSZ) TBCs produced by electron beam physical vapor deposition(EB-PVD) were exposed to CMAS deposits at 1250℃.The microstructure evolution and failure mechanism of the coatings were investigated.It has been shown that CMAS glass penetrated into the YSZ ceramic layer along the inter-columnar gaps and interacted with YSZ.As a result,an interaction zone of about 20μm thickness,which was the mixture of CMAS and YSZ with equiaxial structure,was formed in the YSZ surface layer after 4h heat-treatment at 1250℃.Meanwhile,yttria in YSZ layer as a stabilizer was dissolved in CMAS glass and caused accelerated monoclinic phase transformation.After 8h heat-treatment,degradation of YSZ TBC occurred by delamination cracking of YSZ layer,which is quite different from the traditional failure caused by interfacial cracking at the YSZ/metallic bond coat.Physical models have been built to describe the failure mechanism of EB-PVD TBCs attacked by CMAS deposits.